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AEgIS

AEgIS (Antimatter Experiment: Gravity,
Interferometry, Spectroscopy) is a physics experiment that takes place at the
european laboratory CERN, using the antiprotons delivered by
the ADaccelerator. AEgIS
is a collaboration of physicists from all around the world.

The primary scientific goal of the AEgIS
experiment is the direct measurement of the Earth’s gravitational
acceleration g on antihydrogen. In the first phase of the experiment, a gravity
measurement with 1% precision will be carried out by sending an antihydrogen
beam through a classical Moire deflectometer coupled to a position sensitive
detector. This will represent the first direct measurement of a gravitational
effect on an antimatter system.

A new generation of
antimatter experiments has been opened after the first experiments of
antihydrogen production in laboratory conditions at the CERN laboratory [1] and
at Fermilab [2]. These experiments produced hot , i.e.
relativistic, in small quantities not suited to precision studies.
Thereafter a program is underway at CERN with a facility dedicated to low
energy and experiments. After the first production of
cold by the ATHENA [3, 4] and ATRAP [5, 6] collaborations, second
generation experiments, as ALPHA [7] and ASACUSA [8], are being performed for
measuring the fundamental properties of this antiatom. AEgIS is an experiment
approved by CERN with the goal of studying physics [9].
Some fundamental questions of modern physics relevant to unification of gravity
with the other fundamental interactions, models involving vector and scalar
gravitons, matter–antimatter symmetry can be enlightened via experiments with
antimatter [10]. A quantum theory of gravitation necessarily constitutes a
departure from the Einstein view of gravity as a geometric phenomenon and could
potentially constitute a violation of the weak equivalence principle. This
principle is a foundation of General Relativity and a large experimental effort
is placed in testing its consequences in all possible fields: this research
activity includes tests of the equality of the inertial and gravitational mass,
the universality of the free fall, the search for non Newtonian corrections to
the gravitational law, the measurement of the gravitational red shift, and the
search for time variation of the fundamental constants. Measurements studying
the equality of the inertial and gravitational mass of different macroscopic
bodies or cold atoms have only been performed on ordinary matter. There are no
direct measurements about the validity of the principle of equivalence for
antimatter. At present, the validity of the equivalence principle for
antimatter is extrapolated from the matter results or it is inferred using
indirect arguments. Particularly interesting is that some quantum gravity
models leave room for possible violations of the equivalence principle for
antimatter [11]. Modern theories of gravity that attempt to unify gravity with
the other forces of nature allow that, at least in principle, antimatter may
fall differently from normal matter in the Earth‘s gravitational field.
Specifically, as pointed out by Sherk [12], theories of supergravity are open
to the possibility of a gravitational interaction which have different
couplings for matter and antimatter.
The recent production of copious amounts of cold antihydrogen at
CERN’s Antiproton Decelerator (AD) [3, 13] has paved the way for high-precision
gravity experiments with neutral antimatter. In the first phase of the
experiment, acceleration in a controlled way by an electric field gradient
(Stark effect) and subsequent measurement of free fall in a Moiré deflectometer
will allow a test of the weak equivalence principle. In a second phase, the antihydrogen
will be slowed, confined and laser-cooled to perform CPT studies and detailed
spectroscopy.

Figure 1.
Sketch of the experimental setup region where antiprotons and positrons are
manipulated to form and to accelerate antihydrogen. This region is at low
temperature (~100 mK) in an axial magnetic field of 1 T (after Ref. 14).

Figure 1 shows a schematic drawing of the basic experimental setup that should
reach an accuracy of 1% in the measurement of the matter-antimatter gravitational
acceleration. The experiment is designed to allow higher precision measurements
through radial cooling of the beam [15]. The essential steps leading to the
production of antihydrogen (Hbar) and the measurement of its gravitational
interaction in AEgIS with the use of CERN cold antiproton (pbar) are the
following: i) accumulation of positrons (e+) in a Surko-type source and trap
[16]; ii) capture and accumulation of pbar from the AD in a cylindrical
Penning-Malberg trap [17]; iii) cooling of the pbar cloud to sub-K
temperatures; iv) production of cold positronium (Ps) by bombardment of a
cryogenic nanoporous material with an intense e+ pulse; v) two-steps laser
excitation of Ps to a Rydberg state (Ps*) with principal quantum number n >
20; vi) pulsed formation of cold Rydberg antihydrogen by means of the resonant
charge exchange interaction between Rydberg Ps* and cold pbar with a residual
electron; vii) pulsed formation of an Hbar beam by Stark acceleration with
inhomogeneous electric fields; viii) determination of g in a two-grating Moiré
deflectometer coupled with a position-sensitive detector.

The formation and excitation of Ps to a Rydberg state represent a very
interesting challenge. Our proposed technique of formation is
conceptually similar to a charge exchange technique based on Rydberg Cs [18]
which has been successfully demonstrated by ATRAP [19], but offers greater
control of the final state distribution of and allows pulsed
production of .
Construction has started on the AEgIS experiment, whose design is based upon
the broad experience gained with the ATHENA and ATRAP experiments at the AD, a
series of ongoing related experiments, tests and developments, as well as
extensive simulations of critical processes (charge exchange production of ,
Stark acceleration and propagation through the Moiré deflectometer, resolution
of the position-sensitive detector located at the end of the deflectometer).
The proposed gravity measurement merges in a single experimental apparatus
technologies already demonstrated or based on reasonable additional
development.
For the initial phase of the experiment, obtaining samples of anti-atoms at 100
mK is an essential requirement. Gravity measurements with even higher
precision, as well as competitive CPT tests through spectroscopy, are
desirable, but will necessitate the development of novel techniques to attain
even colder ensembles. The experiment has been designed with
flexibility of the apparatus in mind, in order to allow a number of techniques,
which may lead to such physics topics, to be implemented. One natural extension
of the modular design is to incorporate, in a future stage, a magnetic
decelerator and trap for , which will be spatially separated from the region
where the anti-atoms are produced, similar to the devices currently being used
to trap and study H atoms [20, 21]. The experience gained in the first phase of
AEgIS with the formation of a beam will be used to optimize the
design of such a trapping system.